This invention relates generally to manufacturing fixtures and more particularly to a method and device for accommodating variations from acceptable tolerances in the manufacture or configuration of parts to be joined by ultrasonic welding.
Ultrasonic welding joins parts by applying mechanical energy from a high frequency vibration source at a joint formed between two parts. Generation of ultrasonic energy includes the conversion of high frequency electrical energy by an electromechanical converter or transducer to a mechanical vibration. Mechanical vibrations are amplified by an amplitude transformer and applied to a workpiece by means of an ultrasonic xe2x80x9chornxe2x80x9d. The mechanical vibration is produced at a frequency typically in the range of 20,000 (20 kHz) to 40,000 (40 kHz) cycles per second. The high frequency vibration is ordinarily accompanied by a compressive force generated by the horn between the mechanically vibrated part and the relatively stationary part.
During the ultrasonic welding process, the relatively stationary part is typically positioned in a fixture, often referred to as an anvil or nest. The fixture is most often configured to support and conform at least in part to a configuration of the relatively stationary part thereby providing a means to capture the part and eliminate lateral motion during the welding process. Fixtures therefore tend to be part specific. Additionally, the fixture is commonly attached rigidly to a work surface and positioned relative to the ultrasonic horn so as to accommodate positioning of the parts and performance of the welding process.
The vibrated part is mechanically vibrated and compressed against the relatively stationary part joining the parts at an interface. The ultrasonic welding process is capable of breaking down films and oxides creating a coalescence between the parts at the site of the weld. A superior joint may be achieved without the use of filler materials, fluxes, adhesives, epoxies or mechanical connectors.
One parameter deemed critical to the ultrasonic welding process is the fit at the interface between parts. Process parameters often specify acceptable tolerances for fit at the interface between parts to be joined. Due to the part specific nature of the fixture, the rigid attachment of the fixture in the process environment and the process critical relationship of fit and interface between parts to be joined, the ultrasonic welding process may be unforgiving where parts are manufactured even slightly out of tolerance.
There may be advantage in providing a self adjusting fixture for use in an ultrasonic welding process. The self adjusting fixture may accommodate for variations from acceptable tolerances in the manufacture of parts.
The present invention is directed to a method and device for permitting, by self adjustment, part interface variations. By permitting part interface variations it is meant accommodating variations in the fit at the interface between parts to be joined using an ultrasonic welding process. Variations in the fit at the interface may be caused by variations in the manufacture or configuration of the parts to be joined, or variations in the manufacturing process itself wherein the parts are fit together in preparation for the ultrasonic welding process. In many applications, particularly in those instances where hermetically sealed welds are required, adjustment to fit at the interface of the parts to be joined may result in a desirable improvement.
In one embodiment, the self adjusting fixture for accommodating part interface variation includes a part support member including a first self adjusting member engagement element. The self adjusting fixture also includes a pedestal having a second self adjusting member engagement element engageable with the first self adjusting member engagement element. The self adjusting members permit axial deflection of the nest relative to the pedestal about a primary axis.
In one preferred embodiment of the invention, the self adjusting fixture is configured as an anvil having a nest including a hemispherical socket formed in an underside of the nest. The anvil also includes a pedestal mountable to a base member. The pedestal includes a hemispherical distal end which is sized for insertion within the socket of the nest to permit axial deflection of the nest relative to the pedestal. Preferably, the compressive force exerted during the welding process should lie co-axially to the primary axis and therefore to the axis of axial deflection of the nest relative to the pedestal.
In one preferred embodiment of the invention, axial deflection of the nest relative to the pedestal is limited by a deflection limiting element. The deflection limiting element may include a physical feature of the nest which interacts with a physical feature of the base or the pedestal to limit axial deflection of the nest relative to the pedestal.. The deflection limiting element may include a flat circular element disposed about the pedestal that restricts angular deflection of the nest in three hundred and sixty degrees. Alternately, axial deflection of the nest relative to the pedestal may be limited by a corresponding fit between a shaft of the pedestal and a segment of a wall of the first engagement element.
Rotation of the nest about the axis of deflection relative to the pedestal is limited by a rotation limiting element. The rotation limiting element may include a physical feature of the nest which interacts with a physical feature of the base or the pedestal to limit rotation of the nest about the axis of deflection. In one preferred embodiment of the invention, a pin extends from the nest along an axis substantially perpendicular to the first axis. The pin is captured by a pair of fingers extending from a base member. In the event that rotation of the nest is initiated, such rotation is limited by the pair of fingers.
A method for self adjustment for part interface variations includes the steps of supporting a relatively stationary part including a first interface element on a support member including a self adjusting member first engagement element and a pedestal including a self adjusting member second engagement element axially engaged to the self adjusting member first engagement element, the support member axially deflectable about a first axis, fitting a vibrated part including a second interface element to the first interface element of the relatively stationary part and applying a compressive force between the relatively stationary part and the vibrated part substantially along the first axis for axially deflecting the relatively stationary part with respect to the vibrated part to achieve improved fit between the first interface element and the second interface element.